With the progress of technology, various electronic products have been developed for stimulating the growth of the information technology market. Undoubtedly, such trend will carry on. Also, with the advancement of the microelectronic technology, the electronic products will be more versatile and more miniaturized. Besides, the portability of the electronic products will be enhanced as well. Nowadays, the user can handle all kinds of business easily with various electronic products. In recent years, the so-called piezoelectric actuator have been developed and applied to electronic products. The piezoelectric actuator have the advantages of low voltage, high immunity to noise, small size, fast response, low heat radiation, high sophistication, high conversion efficiency, and high controllability.
The piezoelectric actuator generally requires an AC voltage that is applied thereto to drive the piezoelectric actuator to carry out high-speed periodically operations. Hence, the piezoelectric actuator needs a driving system to operate. The driving system is used to convert a DC voltage into an AC voltage for driving the piezoelectric actuator. Referring to FIG. 1, the conventional driving system 1 is used to convert a DC input voltage VDC into output AC voltages Vo1 and Vo2 for driving a piezoelectric actuator 9 shown in FIG. 2A. The driving system 1 includes a boost circuit 10, a voltage multiplier 11, and a polarity switching circuit 12. The boost circuit 10 is connected to the DC input voltage VDC to convert the DC input voltage VDC into a transient voltage VT by the switching operations of the internal switch elements and the energy storage and filtering operations carried out by the internal inductors, capacitors, and diodes. The voltage multiplier 11 is connected to the transient voltage VT to multiply the transient voltage VT by 4 to generate a DC high voltage VB. The polarity switching circuit 12 is used to convert the DC high voltage VB into output AC voltages Vo1 and Vo2 for driving the piezoelectric actuator 9.
Referring to FIGS. 2A, 2B, and 3 with reference to FIG. 1, in which FIG. 2A shows the internal circuitry of the polarity switching circuit of FIG. 1 and FIG. 2B illustrates the operation of the polarity switching circuit of FIG. 1 as the digital signal fsw is low. Also, FIG. 3 shows the timing of the voltage signals of FIG. 2A and FIG. 2B. The polarity switching circuit 12 is connected to the DC high voltage VB, the input DC low voltage Vin, and the digital signal fsw to convert the DC high voltage VB into output AC voltages Vo1 and Vo2 driving the piezoelectric actuator 9 to operate repetitively. The polarity switching circuit 12 includes a first current-limiting resistor R21, a second current-limiting resistor R22, a third current-limiting resistor R23, a first transistor switch Q21, a second transistor switch Q22, a third transistor switch Q23, a fourth transistor switch Q24, a fifth transistor switch Q25, a sixth transistor switch Q26, and a seventh transistor switch Q27.
As the digital signal fsw is high and is sent to the control terminal of the first transistor switch Q21 and the control terminal of the sixth transistor switch Q26, the first transistor switch Q21 and the sixth transistor switch Q26 that are connected to the ground terminal G will turn on. As the first current-limiting resistor R21 is connected to the first transistor switch Q21, the circuit branch consisted of the first current-limiting resistor R21 will be connected to the ground terminal G. Meanwhile, the second transistor switch Q22 and the fourth transistor switch Q24 will turn off as the control terminal of the second transistor switch Q22 and the control terminal of the fourth transistor switch Q24 are connected to the circuit branch consisted of the first current-limiting resistor R21, thereby driving the voltage level of the circuit branch consisted of the second current-limiting resistor R22 to a high level due to the DC high voltage VB. hence, the third transistor switch Q23 will turn on as the control terminal of the third transistor switch Q23 is connected to the circuit branch consisted of the second current-limiting resistor R22. Meanwhile, the control terminal of the seventh transistor switch Q27 is connected to the digital signal fsw with a high level. Therefore, the seventh transistor switch Q27 is also turned on. As the third current-limiting resistor R23 is connected to the seventh transistor switch Q27, the circuit branch consisted of the third current-limiting resistor R23 is connected to the ground terminal G. Also, the control terminal of the fifth transistor switch Q25 is connected to the circuit branch consisted of the third current-limiting resistor R23, the fifth transistor switch Q25 is turned off. Therefore, the current will flow in the direction as indicated by the arrows shown in FIG. 2A.
As the digital signal fsw is low, as shown in FIG. 2B, the operations of all the transistor switches are reverse to the operations of all the transistor switches indicated in FIG. 2A. Under this condition, the current flow will be indicated by the arrows shown in FIG. 2B. In this manner, the output AC voltages Vo1 and Vo2 of the polarity switching circuit 12 will have a square waveform on the piezoelectric actuator 9, as indicated by the waveform of the voltage signal of (Vo1-Vo2) shown in FIG. 3.
As the output AC voltages Vo1 and Vo2 of the polarity switching circuit 12 have square waveforms on the piezoelectric actuator 9, the piezoelectric actuator 9 is rapidly charged as the voltage levels of the output AC voltages Vo1 and Vo2 are bobbing rapidly. Although the piezoelectric actuator 9 can reach the peak of its amplitude due to the rapid charging of the piezoelectric actuator 9, the power loss is increased as well. More disadvantageously, as the polarity switching circuit 12 is configured to charge the piezoelectric actuator 9 rapidly with square AC waves, the piezoelectric actuator 9 will vibrate under a natural resonant frequency. Such vibration will cause tremendous noise.
Hence, it is needed to develop a polarity switching circuit to address the problems encountered by the prior art. The invention can meet this need.